Combustion And Gasification Application Of Biomass Pyrolysis Oil:Measurement And Modeling

With the rapid development of human society, increasing environmental concerns and the rising demand for fossil fuel have increased the necessity in developing and manufacturing alternative source of energy, which is renewable in nature. Biomass is the only renewable carbon source that can be storage and handling. Solid Biomass can be converted into carbon-rich liquid bio-oil, solid chars and non-condensable gas through a process called pyrolysis. The chemical and physical properties of bio-oil have much more differences with traditional fossil fuel and cannot dissolve each other. The most simple and feasible application of bio-oil is combustion. The most promising refined technology of bio-oil is to produce syngas via gasification, after that syngas is used to synthesize high-grade vehicle fuel. Based on this background, a series of experiments and numerical simulation were presented to integrally investigate bio-oil spraying properties, combustion and gasification characteristic. The bio-oil combustion and gasification plants were design and built.Thermal weight loss curves of bio-oil and its blends with low molecular weight alcohols were determined by TG-DTA method in the oxygen atmosphere and compared with traditional fossil fuel diesel and heavy fuel oil. The thermo gravimetric cures of bio-oil and its blends with alcohols can be divided into three steps:volatilization, secondary char formation, and char combustion. With the increasing of alcohols content in the blend fuels, the active energies of volatilization and char combustion both decreased first and then increased. The mass fraction of alcohols should not exceed26%, otherwise the active energy of char combustion increased and the contained organic function groups in char decreased, which indicated higher stability and lower combustibility of the char. The volatility and char combustibility of bio-oil is better than heavy fuel oil.The spray characteristics of bio-oil for combustion were investigated with Phase Doppler Anemometry (PDA) with Sauter mean diameter (SMD) as evaluating indicator. The results showed that SMD of bio-oil spray droplets exhibited a tendency to first decrease and then increase along axial direction and increased continuously along radial direction. The SMD diminished with the increase of gas-liquid flow ratio and had little variation when the gas-liquid flow ratio exceeded a certain value. Among three kinds of fuels, the atomizing quality of diesel was the best, while that of the bio-oil was the worst. By adding methanol or ethanol to the bio-oil, the atomizing quality was improved. The empirical equation for the SMD was derived. The predicted results were found to be in a good agreement with the experiment results.FLUENT was used to simulate the combustion process of bio-oil and heavy fuel oil with the same heating value flow in a swirl combustion chamber. Combustion and emission characteristics of them were comparative study to provide useful data for the application of bio-oil, and the effects of swirl number on the bio-oil combustion were simulated. The results indicate that there existed central recirculation region, the recirculation area and flow of bio-oil combustion were smaller than heavy fuel oil combustion. The maximum furnace temperature of heavy fuel oil combustion is300K higher than bio-oil. The predicted gas species distributions of heavy fuel oil combustion were found to be in a reasonable agreement with the measurements, and heavy fuel oil release more CO and NO than bio-oil combustion in the flue gas. With the increase of swirl number, the recirculation area and flow increased, and the formation of CO can be effectively reduced, but the NO concentration in the outlet increased at first and then decreased. Bio-oil can replace heavy fuel oil combustion in the boiler.Three-dimensional numerical simulation performed to obtain detailed information to reveal the characteristics of the flow, combustion, heat and mass transfer process in the kiln. The simulation results agreed well with the experimental results, which confirmed the reliability of the model. According to the experimental and simulated results, the kiln experimental combustion system optimized for reducing size and simplifying structure, a new atomizing nozzle employed. Combustion experiments and simulation results show that atomizing air flow rate achieving the same spraying quality significantly reduces. Although the oil flow rate decrease, measured combustion temperature (exceeds1450K) is higher than original kiln. Changing the atomizing air flow rate can be an optimizing factor in improving atomizing quality and liquid-oxygen mixing in central axis lead to short residence time to burn-out. The minimum CO and maximum NO concentration in flue gas obtained when air atomizing flow rate is2.0kg/h. With the increasing of equivalence ratio, the reacting oxygen concentration, turbulence and combustion quality is increased, combustion zone get downstream. The CO concentration in flue gas inhibited, while the formation of NO is increased. The optimum equivalence ratio is 1.22that CO and NO emissions both within a low level. The predicted temperature, CO and NO concentrations were in a reasonable agreement with the experiment data, which confirmed the reliability of the model.Based on experimental and simulation results, taking into account the fuel characteristics of bio-oil, a bio-oil combustion device was build. The bio-oil could be ignited while the combustion chamber was preheated up to about250℃and existed naked flame. The flame was not stable at the beginning because the temperature of furnace was too low to evaporate bio-oil quickly. With the increase of burn time, the flame move downstream. After25min, the temperature at location1.5m from injection slightly increased from900K to1350K, the combustion flame became steady and the pollutant emissions were lower than emission standard.The thermo gravimetric cures of bio-oil in the nitrogen atmosphere can be divided into two steps:volatilization and carbonization. The increasing of heating rate can reduce the formation of carbon, increase gasification efficiency. A bio-oil gasification device was build, a serious of experiments and simulation by FLUENT were presented. The experimental and predicted temperatures in gasification reactor are in reasonable agreement. The carrier gas N2inhibit the carbonization reaction between bio-oil components, but the high flow rate of carrier gas lead bio-oil gasification reaction time and gasification efficiency reduce. The gasification reaction time need be more than1s. With the increase of equivalence ratio, the combustible gas production decrease firstly and then increase, CO2production and carbon conversion increase because the oxidation reaction increase the gasification temperature. The experimental and simulation results indicate that part of the bio-oil complete combustion is the key to achieve effectively auto thermal gasification.The economic analysis of biomass pyrolysis technology proves that the technology has a good economy for industrialization. The combustion application of bio-oil as a substitute for fossil fuel oil has good economic benefits. In a large Fischer-Tropsch synthesis plant, the cost of bio-oil gasification to produce synthesis gas is lower than biomass gasification. The production and utilization of bio-oil will bring significant social and environmental benefits.